Heretofore, video clamping circuits used in satellite communication systems have employed a configuration as shown in FIG. 1. By referring to FIGS. 1-3, the problems that the prior art circuit have encountered are discussed below.
Referring first to FIG. 1, the circuit includes a clamping capacitor 1, a clamping diode 2, a transistor 3, an emitter resistor 4, a capacitor 5 for cutting off DC components, bias resistors 6 and 7, and bypass capacitors 8 and 9. It will be understood later that a device similar to the transistor 3 constitutes the buffer amplifier of a circuit according to the present invention.
An input signal (FIG. 5(A)) consisting of a video signal on which an energy dispersal signal in the form of a triangular waveform of 30 Hz, for example, is superimposed is applied to input terminal IN. This input signal is shown in FIG. 2(A) more accurately. A small value is selected for the capacitance of the capacitor 1 so that whenever a horizontal sync signal is produced the diode 2 is triggered into conduction. In particular, electric charges stored in the capacitor 1 in a polarity relation as shown are discharged by a parallel resistance which is the combination of the input resistance of the emitter follower circuit formed by the transistor 3 and the resistance produced when the clamping diode is reverse-biased. The time constant of the discharge is so determined that the discharge occurs substantially with the period of the horizontal sync signal by appropriately selecting the capacitance of the capacitor 1.
In this way, by selecting a small value for the capacitance of the capacitance 1, it is possible to clamp the potential of each leading edge of the horizontal sync signal shown in FIG. 2(A) at a constant value in synchronism with the ocurrence of the sync signal. Therefore, any low frequency component like an energy dispersal signal synchronized with a vertical sync signal can be eliminated. The aforementioned clamping at a constant potential means that the potential at point A is maintained at a given potential while the diode 2 is in a conduction state. That is, in this state of the diode 2, the voltage V.sub.B of a B-power supply is divided down to a voltage K.multidot.V.sub.B by the resistors 6 and 7, and a voltage V.sub.f applied to the diode 2 in the forward direction is subtracted from the voltage k.multidot.V.sub.B with the result that a constant voltage is applied to the point A.
The capacitor 1 should function to pass a video signal, and an ideally passed waveform is shown in FIG. 3(A). However, a small capacitance of the capacitor 1 introduces a distortion in the output waveform of the video signal as shown in FIG. 3(B). This phenomenon will be readily understood from the fact that an excessively small capacitance of the capacitor 1 deteriorates its low frequency characteristics. Consequently, it is required that a relatively large value be selected for the capacitance of the capacitor 1, but as the capacitance is increased, the aforesaid energy dispersal signal is less effectively removed. This is discussed in more detail in the following.
The deterioration in the percentage of the removal depends considerably on the fact that the diode 2 exhibits a forward direction characteristic as shown in FIG. 2(D). Specifically, if the capacitance of the capacitor 1 is relatively large, then the capacitor 1 will not be discharged completely at the instant that the horizontal sync signal is introduced, thus rendering current If flowing through the diode 2 in the forward direction relatively small. Therefore, there exists a difference .DELTA.If between the current value of the triangular waveform when it rises and the value when it falls as shown in FIG. 2(B). This causes the voltage Vf applied to the diode 2 to change by a relatively large value .DELTA.Vf(B) as shown in FIG. 2(D). As a result, the potential at point A of FIG. 1 varies comparatively greatly between the instant at which the waveform is on the increase and the instant at which it is on the decrease.
Conversely, if the capacitance of the capacitor 1 is relatively small, the capacitor is completely discharged until the horizontal sync signal is introduced and so the current If flowing through the diode 2 in the forward direction is relatively large. By this reason, even if there exists the aforementioned current value difference .DELTA.If in the triangular waveform between the two points as shown in FIG. 2(C), the voltage Vf varies by a smaller value .DELTA.Vf(C) as shown in FIG. 2(D). As such, the variation in the potential at the point A of FIG. 1 is smaller.
Accordingly, the percentage of removal of the energy dispersal signal must be determined by making a trade-off with the waveform distortion. For the waveform distortion as shown in FIG. 3(B), vertical sync distortion and so forth should be held within a negligible level, say 5%. A satisfactory result will not result, because experiment shows that the ratio of the removed dispersal signal is simply on the order of 26 dB. The minimum detectable value of the ratio is 35 to 40 dB, and general specifications require 40 dB.